This invention relates to a comparator.
A conventional comparator generally has a structure such as that shown in FIG. 3 in which a transistor T.sub.3 is connected in series with a resistor R.sub.3, a transistor T.sub.4 is connected in series with a resistor R.sub.4, the junction of transistor T.sub.3 and resistor R.sub.3 and the junction of transistor T.sub.4 and resistor R.sub.4 are connected to inputs of a differential amplifier A which produces an output corresponding to the difference between the signals supplied to the bases of transistors T.sub.3 and T.sub.4.
In this arrangement, setting a comparison level is performed by changing a reference potential in the case of a single input while it is performed by changing resistors R.sub.3, R.sub.4 in the case of a differential input.
The setting of the comparison level in the case of the differential input will now be described. If the difference between the inputs to the bases of transistors T.sub.3 and T.sub.4 is represented by Vi, and a change in the current is represented by i, currents (I+i), (-i) flow through transistors T.sub.3 and T.sub.4, respectively, and the respective collector potentials V.sub.t1c, V.sub.t2c are given by EQU V.sub.t1c =V.sub.cc -R.sub.3 (I+i) EQU V.sub.t2c =V.sub.cc -R.sub.4 (i-i)
Since V.sub.t1c =V.sub.t2c, the comparator starts to operate when the comparison level is EQU i=I(R.sub.4 -R.sub.3)/(R.sub.3 +R.sub.4)
It can be seen from this equation that when, for example, R.sub.3 =R.sub.4, the comparator operates with i=0, while when R.sub.4 =2 R.sub.3, the comparator operates with i=I/3. Namely, when R.sub.4 =2R.sub.3, the comparator operates with application of an operating input voltage v.sub.1 thereto such that a current i=I/3 flows therethrough.
A drawback in the above circuit arrangement is that when the comparison level is set with a reference voltage, the reference voltage and an input potential to be compared must be of high accuracy.
In the case of a differential input, as described above, the comparison level is determined by (R.sub.4 -R.sub.3)/(R.sub.3 +R.sub.4), so that disadvantageously the relative accuracy of the resistors is required. Especially, when the resistances of resistors R.sub.3 and R.sub.4 are different, integration is difficult.